Abstract: The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a space science project proposed by the Chinese Academy of Sciences specifically for exploring the significant opportunity of the gravitational wave multi-messenger astronomy by monitoring the gravitational wave high energy electromagnetic counterpart in all-sky. GECAM satellite can downlink the EVENT data, housekeeping data, and alarm data through X-band, S-band and also BeiDou-3. The ground segment preprocessing process needs to process these data quickly and correctly to meet scientists requirement for correctness and timelines. In this paper, we design the scientific data processing flow and the astronomical warning information processing flow according to the characteristics of GECAM data processing, and also we outline the key core algorithms and show the implementation of the key methods. After the satellite is successfully launched, the data preprocessing software runs well, and the accuracy meets the system requirements. The work done in this paper provides guidelines for the follow-up Einstein Probe (EP) satellite, Sino-French Astronomical Satellite (SVOM) and other space astronomical satellites.